Optical tunable grid-assisted add/drop filter in codirectional mode of operation

ABSTRACT

A tunealble optical grid-assisted add/drop filter for codirectional operational mode consisting of at least two waveguides composed of two different classes of material of different optical parameters. The thermal refractive index coefficient dn/dT, the electro-optic coefficient dn/dE or the dispersion dn/dλ of the two materials differ from each other such that when the temperature, the electric field or the wavelength of or in the two waveguides is change, the result will be effects of different powers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical tunealble grid-assisted add/dropfilter in codirectional mode of operation structured as a directionalcoupler filter with at least two waveguide extending closely adjacent ofdifferent refractive indices, one of the waveguides being provided witha grid.

2. The Prior Art

In optical telecommunications technology add/drop filters are keycomponents in so-called WDM (wavelength division multiplexing) systemsin which several wavelengths are propagated in one fiber strand. Suchadd/drop filters adding and dropping one or more wavelengths into orfrom the fiber. It is known to structure add/drop filters as directionalcoupler filters. In a filter of this kind there are arranged at leasttwo waveguides extending closely adjacent each other and of differentrefractive indices, at least one of the waveguides being provided with agrid. Preferably, the grid is disposed on the waveguide of the higherrefractive index. Such an add/drop filter has been described, forinstance, in IEEE Phot. Technol. Lett., Vol. 4, No. 12, December 1992,pp. 1386-1389.

Purely fiber-optical realizations of add/drop filters in which silica isused for the fibers, have been described in IEEE Phot. Technol. Lett.,Vol. 8, No. 12, December 1996, pp. 1656-1658 and in U.S. Pat. No.5,978,530, the latter publication referring to the possibility ofrealization in planar optical geometry. However, these fiber-opticalfilters are not structured for tuning.

The prior art is familiar with tuneable filters structured as commonMach-Zehnder interferometers with a poorly selective sin²-filter curve.Such filters may be tuned, for instance, by separate thermo-opticallyeffective heating electrodes mounted on the waveguide or guides.

In InP-technology comparable filters are known which may be detunedthermo-optically or electro-optically. For instance, a thermo-opticallydetunable filter has been described in Proc. of the 10^(th) Int. Conf.on InP and Rel. Mat., (IPRM '98), Tsukuba, Japan, (1998)pp.7—Post-deadline paper. The filter disclosed is made of materials of aclass (III-V-compounds), i.e. GaInAsP/InP, as an asymmetric lateralgrid-assisted codirectional direction coupler filter of two waveguides,one of which is weakly propagating and one of which is stronglypropagating, with the grid being disposed on the strongly propagatingwave gate and resulting in a wavelength selective behavior of thecomponent. However, high manufacturing tolerances are required forfabricating small dimensions of the mentioned filters in InP technology;also, there is a higher attenuation as a consequence of the modemismatching relative to glass fibers.

The prior art (see Electr. Lett. Vol. 36, No. 5, pp. 430-431, 2000) alsodiscloses an arrangement in which a thermo-optical coupler switch isprovided with two parallel SiO₂ waveguides on which is disposed apolymeric waveguide such that it intersects the SiO₂ waveguides disposedbeneath it. At each intersection with a SiO₂ waveguide the polymericwaveguide is provided with a heating electrode. In this case, the planarSiO₂ waveguide is utilized as a transmissive layer and the polymericwaveguide is utilized for the switching function.

OBJECTS OF THE INVENTION

It is the object of the invention to provide a high-resolution tuneableadd/drop filter of a spectral band width of the filter transmissivecurve in the range in excess of 50 GHz, which can be fabricated in asimpler and more cost-efficient manner with higher dimensionaltolerances compared to add/drop filters based on III-V materials.

It is to be noted that band width ranges between 50 GHz and 400 GHz areof particular interest to current and future applications in the fieldof communications technology.

SUMMARY OF THE INVENTION

The object is accomplished in an add/drop filter of the kind referred toabove by the material of the two waveguides consisting of two differentmaterial classes of different optical parameters with the coefficient ofthe thermal refractive index dn/dT and/or the electro-opticalcoefficient dn/dE and/or the dispersion dn/dλ of the two materialsdiffering such that by acting on the two waveguides with the sametechnical means for changing the temperature and/or the electric fieldand/or the wavelength effects of different strengths result, and bymeans being provided for changing these optical parameters.

In the known arrangement of an add/drop filter based on a directionalcoupler structure in which two parallel waveguides are separated by agap over a certain length, the two waveguides are differentlydimensioned (e.g. width, height, refractive index) as a result ofmanufacturing tolerances and deliberate settings. This results indifferent propagation constants within the waveguides, i.e. thecomponent is asymmetric. At such asymmetry a complete exchange of energybetween the two waveguides is no longer possible. Symmetry may again beestablished by applying a grid to one of the waveguides. The highfrequency selectivity connected therewith is utilized for the filteringfunction. The waveguides of an add/drop filter which in accordance withthe invention are made of materials from two different classes inwhich—as has been mentioned—the optical parameters differ from eachother, make possible deliberate tuning of the filter by simple technicalmeans.

An embodiment of the add/drop filter in accordance with the inventionprovides for one waveguide being made of silica and the other waveguidebeing made of a polymeric material, more particularly a polymericmaterial of non-linear optical properties.

In another embodiment of the invention the waveguide with the largerrefractive index is provided with a grid.

In further embodiments the two waveguides are arranged vertically orhorizontally with respect to each other. The realization of the twowaveguides in laminar planar micro technology in particularly results ina simple and cost-efficient fabrication of the entire filter.

Since in accordance with the invention the coefficient of the thermalrefractive index dn/dT of the two waveguides made of materials from twodifferent classes differs such that at an identical change oftemperature the reaction of the two waveguides in respect of theirrefractive indices differs, the means for changing the coefficient ofthe thermal refractive index dn/dT is a device for changing thetemperature which acts on the entire surface of the chip, particularlyfrom below. More particularly, the device may be identical to a deviceof the kind, e.g. a Peltier element, commonly used for stabilizing thetemperature of known add/drop filters. This embodiment requires noadditional step for fabricating specific heating electrodes andappurtenant controls, and at a homogeneous change in temperature in theentire component it makes possible different changes in the refractiveindices of the two waveguides. This results in an effective thermaltuning of the component without an application of local heatingelectrodes. In this manner, the invention—compared to the arrangementsof the prior art—makes possible temperature stabilization andtemperature trimming for balancing manufacturing tolerances as well asthe desired wavelength tuning of the filter.

Of course, in a different embodiment of the invention the means forchanging the coefficient of the thermal refractive index dn/dT may be aheating electrode disposed on the waveguide the coefficient of whichdisplays a greater temperature dependency.

The means for changing the optical parameter dn/dE are electrodes forgenerating an electric field E with at least one electrode beingdisposed on the waveguide having the greater index of refraction. If anelectric field is applied to this waveguide, it is known that inaccordance with the Pockel effect the refractive index of the polymer(Δn˜E) will change. The specific arrangement of the electrodes forgenerating an electric field which affects the refractive index of theunderlying waveguide will be decided by the person skilled in the art independence of the desired direction of the electric field and the activeelectro-optic coefficient for affecting the mode coupling in apredetermined manner. For instance, a vertical electric field E_(I) isgenerated by an electrode arranged directly on the NLO polymer waveguideto make use of the greatest electro-optic coefficient for polymericwaveguides having non-linear optic properties. In the present case, theelectro-optic coefficient r₃₃ for TM polarization is utilized. Ahorizontal electric field E_(I) is generated by electrodes disposed atboth sides of the polymer waveguide. In this case, use is made of theelectro-optic coefficient r₁₃. In general R₁₃≈⅓r₃₃.

Polymers which have a great electro-optic coefficient dn/dE tuning maybe achieved in the ps range. It may thus be realized more quickly thanby a change of temperature and the action by the thermo-optic effect incomparable arrangements. This is true, in particular, in connection withthe non-linear optic polymer materials mentioned above.

It is to be noted that the selected difference between the refractiveindices of the two waveguides be sufficiently large to allow theapplication of a grid known from the prior art. The basic equation forthe grid period Λ of the used grid which is characterized by a periodicchange of the refractive index is: Λ=λ_(c)/(n₁−N₂), wherein λ_(c) is thefiltered wavelength and n₁ and n₂ respectively are the refractiveindices of the strongly and weakly propagating waveguides. As may beseen from the equation, the grid period assumes values if the differencein refractive indices is too small. Thus, the filter would have to bebuilt very long (e.g. 100 mm). This would be disadvantageous. Byincreasing the stroke of the grid (grid amplitude) the length of thegrid may be shortened provided the radiant emissions remain negligible.

A change of the dispersion dn/dλ may be achieved during fabrication ofthe filter in accordance with the invention by selecting other materialsfor making the waveguides.

Tuning of the add/drop filter in accordance with the invention may becarried out by changing individual optical parameters or by acombination thereof.

When fabricating the arrangement in accordance with the invention usinga polymer as waveguide material having, for instance, a refractive indexn₁=1.49, the geometric dimensions are larger by a factor of 3 than anInP (III-V) structure of refractive index of about 3.3. This wouldrender the fabrication technology simpler and more cost-efficient, inview of the fact that higher dimensional tolerances can be permitted.Matching with optical glass fibers which establish the connection to thefour ports (Inputs/outputs) may, without tapering, be of significantlylower attenuation (per unit length) than would be the case withInP-based add/drop filters. For tuning the filters made, in accordancewith the invention, from polymer/silica, a wavelength shift above thetemperature of about 3 nm/K is expected. Publications relating tocomparable filters in InP report a lower sensitivity of 0.37 nm/K.

DESCRIPTION OF THE SEVERAL DRAWINGS

The novel features which are considered to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, in respect of its structure, construction andlay-out as well as manufacturing techniques, together with other objectsand advantages thereof, will be best understood from the followingdescription of preferred embodiments when read in connection with theappended drawings, in which:

FIG. 1 depicts the shift in the filter curve at a temperature increaseof 10 K uniformly acting on the chip provided with the add/drop filterin accordance with the invention;

FIG. 2 depicts the shift of the filter curve upon application of anelectric field to the add/drop filter in accordance with the invention;and

FIG. 3 depicts the effect of the material dispersion on the filterpower.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As is known, an add/drop filter is provided with four gates(Inputs/outputs) connected to each other by two waveguides. One of thewaveguides which extend parallel to each other over a defined length(coupling length) is made of PMMA (polymethylmethacrylate) having arefractive index of n₁=1.49; the other waveguide is made of doped SiO₂(silicondioxide) having a refractive index of N₂=1.454. The twowaveguides are separated by silicon measuring 4 μm in thickness and aredisposed vertically of each other. The waveguide made of polymer is 2 μmthick; the thickness of the one made of doped SiO₂ is 4 μm. Thewaveguide of the greater refractive index is provided with a grid since,as is well known, its effect is more efficient there and the structurallength may thus be shorter. The grid amplitude is 0.2 μm, the gridperiod is 64 μm, and the overall length of the waveguide is 10,600 μm.For a specific optical wavelength the grid affects an energy transfer tobe determined for each grid period. Thus, after a certain number of gridperiods the light signal will completely transfer from one waveguide tothe other. A change in temperature affects the molecular chains of thepolymer which results in a change of the local refractive index and,therefore, of the filter characteristic.

FIG. 1 depicts the shift of the filter curve at a homogeneous increasein temperature by 10 K of the chips provided with the add/drop filter inaccordance with the invention. The tuning behavior as a function of thetemperature was calculated. At an increase in temperature of 10 K (from20° C. to 30° C.) the refractive index of the polymer waveguide changesfrom 1.49 to 1.489. The shift of the frequency towards lower wavelengthsat a temperature increase may be clearly seen. The filter curves showndisplay a sinc characteristic. By apodosaging the grid a Gaussian filtercurve may be attained. In that case the grid amplitude, commencing at 0,will increase to a maximum over the length of the waveguide structureand will thereafter decay again down to 0. Owing to the low grideffectiveness the overall length of the waveguide structure increases bya factor of 3.

It is known that polymeric materials of non-linear optical propertiesalso show a large electro-optical coefficient (dn/dE). This property,too, is utilized in the arrangement in accordance with the invention fortuning the add/drop filter. A change in the refractive index Δn₁ of thewaveguide made of polymeric material is caused by generating, for the TMmode, a vertical electric field E_(I) by an electrode arranged on thepolymeric waveguide and is defined by

Δn ₁=−½n ³ R ₃₃ E,

wherein n₁ is the refractive index of the polymer material and r₃₃ isthe electro-optical coefficient. The values of r₃₃ for electro-opticalpolymers range from about 10 pm/V to about 50 pm/V. An actualcalculation of the refractive index of the polymer material based on thefollowing values n₁=1.49; r₃₃=20 pm/V and thickness of the polymer layerd=5 μm, resulted in accordance with the above equation, upon applying avoltage of 100 V to the polymeric layer, in a change of the refractiveindex of the polymer layer of Δn₁=−6.62·10⁻⁴, Accordingly, therefractive index changed from 1.49 to 1.48934. In FIG. 2, the shift ofthe filter curve is shown as a function of the applied voltage. Applyinga voltage of 100 V results in a shift of the wavelength maximum at aconstant intensity, from 1.5262 μm to 1.5016, i.e. Δλ˜24.6, providedgate 1 functions as the Input and gate 2′ functions as the output, asshown in the inset image. As has already been mentioned, the greattuning speed, relative to the thermo-optic effect, which in principlelies in the ps to sub-ps range and which depends upon the material usedas well as the external circuit arrangement, is of advantage.

Non-linear optical polymers not only display a large electro-opticcoefficient dn/dE but also a large dispersion dn/dλ which near theabsorption length assumes a particularly large value. In the 1.3 μm and1.55 μm range the dispersion values of the non-linear optical polymerschange from −0.02/μm to −0.05/μm. The result of a calculation, shown inFIG. 3, for three different dispersion values demonstrates that thebandwidth of the optical filter depends upon the dispersion. Again, gate1 functions as the input and gate 2′ functions as the output as depictedin the inset image. The 3 dB-bandwidth for the hybrid add/drop filter inaccordance with the invention is, at a dispersion value of −0.012/μm forSiO₂ and −0.01/μm for linear or passive polymers, equal to 4.1 nm. Thisbandwidth changes to 2.5 nm for a nonlinear optical polymer with adispersion of −0.03/μm. If the dispersion is increased to −0.05/μm, the3 db bandwidth is narrowed further to 1.7 nm.

What is claimed is:
 1. An optical tuneable grid-assisted add/drop filterof codirectional operating mode structured as a directional couplerfilter with at least two waveguides of different refractive indices andextending closely adjacent each other, one of the waveguides beingprovided with a grid, characterized by the fact that the material of thetwo waveguides is formed from materials of two different classes ofdifferent optical parameters, the coefficient of the thermal refractiveindex dn/dT and/or the electro-optic coefficient dn/dE and/or thedispersion dn/dλ of the two materials differing such that action on thetwo wave guides of identical technical means for changing thetemperature and/or the electric field and/or the wavelength results indifferent effects and that means are provided for changing these opticalparameters.
 2. The add/drop filter of claim 1, characterized by the factthat one waveguide is made of a polymeric material and the otherwaveguide is made of silica.
 3. The add/drop filter of claim 2,characterized by the fact that the polymeric material posses non-linearproperties.
 4. The add/drop filter of claim 1, characterized by the factthat the means for changing the optical parameter dn/dT is a device forchanging the temperature and acting upon the entire surface of a chipprovided with the filter.
 5. The add/drop filter of claim 4,characterized by the fact that the device for changing the temperatureof the entire surface is identical to a device for stabilizing thetemperature of the chip provided with the filter.
 6. The add/drop filterof claim 1, characterized by the fact that the two waveguides aredisposed horizontally with respect to each other.
 7. The add/drop filterof claim 1, characterized by the fact that that the waveguide with thegreater refractive index coefficient is provided with a grid.
 8. Theadd/drop filter of claim 1, characterized by the fact that the twowaveguides are disposed vertically with respect to each other.
 9. Theadd/drop filter of claim 1, characterized by the fact that the means forchanging the optical parameter dn/dT is a heating electrode disposed onthe waveguide the coefficient of which has a greater temperaturedependency.
 10. The add/drop filter of claim 1, characterized by thefact that the means for changing the optical parameter dn/dE is at leastone electrode for generating an electric field in the waveguide uponwhich the electrode is arranged.